The main issue with most if not all current gearbox systems, either bb or hub mounted, is that you cant change gear under load. Irrelevant of the motor, the moment someone comes up with a relatively lightweight BB base gearbox that can shift under load, and doesn't rely on a grip shift type shifter, it will change the MTB industry overnight.
The Mubea uses this gear box, shifts under load, coasting or stopped and can use a SRAM trigger shifter.
Effigear⎮Bicycle Gearbox⎪Boîte de vitesses vélo⎪France | LA BOÎTE En